Unidirectional condenser microphone



' Dec. 24, 1968 e. NEUMANN 3,418,436

UNIDIRECTIONAL CONDENSER MI CROPHONE Filed July 16, 1965 2 Sheets-Sheet l 1 2 3 L 5 l V an a 22%|? 5 4 PRIOR ART Fig. 1

PRIOR ART Fig. 2

1968 v a. NEUMANN 3, 36

UNIDIRECTIONAL CONDENSER MICROPHONE Filed July 16, 1965 2 Sheets-Sheet 2 ny 5 (A -B) Inventor.-

Geo N umann United States Patent 3,418,436 UNIDIRECTIONAL CONDENSER MICROPHONE Georg Neumann, Winklerstrasse 8a, Berlin-Grunewald, Germany Filed July 16, 1965, Ser. No. 472,419 Claims priority, application Germany, July 21, 1964, N 25,279 8 Claims. (Cl. 179111) ABSTRACT OF THE DISCLOSURE A unidirectional condenser microphone which improves the polar diagram in the frequency region between approximately 6 and approximately 12 kc.p.s., comprising a back plate of the microphone formed with holes extending from the surface of the plate in front of the diaphragm into an acoustical impedance and from this impedance to an opening at the rear of the microphone, and the back plate formed with grooved distributed on its surface in front of the diaphragm, the holes terminating in the grooves, preferably in the crossing points of the grooves.

The present invention relates to a unidirectional condenser microphone and is concerned with the construction of the back plate of a condenser microphone consisting preferably of a diaphragm and of a back plate, which are the two electrodes being insulated from each other.

In all known unidirectional microphones of the velocity or pressure gradient type the back plate arranged behind the diaphragm consists of a plane metallic or metallized plate which has many holes.

Without the holes the compliance of the air volume between the diaphragm and the back plate would be too small. Moreover, at least some of the holes must be deep enough to reach one or more openings of the microphone case, the openings being arranged at some distance behind the area of the diaphragm, to insure that also the rear of the diaphragm may be reached by the sound waves.

In the case of unidirectional microphones, it is important that all sound Waves coming from the rear will reach all points of the front and all points of the rear of the diaphragm with average coincident amplitude and phase. That must be so over the entire frequency range, where the microphone Works according to the pressure gradient or velocity microphone principle.

Theoretically it is not necessary that the diaphragm will stay immovable by sound coming from the rear, but the sum of all positive and of all negative diaphragm movements must be zero.

It is one object of the present invention to provide a condenser microphone wherein a good front-to-back ratio over a wide frequency range may be obtained only, if indeed every part of the diaphragm will stay nearly immovable, when the sound is coming from the rear.

In that case, of course, all points of the front of the diaphragm may be reached very easily by the sound energy; but in order, also to lead the sound energy nearly uniformly to all points of the rear of the diaphragm, usually an air-filled chamber in the form of a short cylinder is arranged inside or straight behind the back plate of capacitive unidirectional microphones. That air-filled chamber fulfills the following tasks:

(1) Combining the sound energy coming through a small number of drilled holes or slits from openings at or near the rear of the microphone and, in that way, usually passing acoustical resistances;

(2) Distributing the sound energy to the great number of drilled holes of the back plate behind the diaphragm;

(3) Increasing the compliance of the air-volume behind 3,418,436 Patented Dec. 24, 1968 the diaphragm which controls the stiffness of the diaphragm.

With this and other objects in view, which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawings, in which:

FIGURE 1 is a cross-section of a known capacitive pressure-gradient microphone with a time delay device disposed behind the diaphragm;

FIG. 2 is a rear elevation thereof;

FIG. 3 is a cross-section of the condenser microphone, designed in accordance with the present invention, shown at an enlarged scale;

FIG. 4 is a front elevation of the microphone disclosed in FIG. 3; and

FIG. 5 is a cross-section of the condenser microphone capsule, disclosing a variation of the friction element.

Referring now to the drawings, the known condenser microphone in FIGS. 1 and 2 comprises a housing 1 to which the diaphragm 1 is secured and which receives a back plate 3. Inside the back plate 3 is an air-filled chamber 5. A plurality of drilled holes 4 extend from the outer surface of the back plate 3 into the chamber 5 and a small number of drilled holes 6 or, instead of that, a number of slits or other air-ducts together with an appropriate acoustical impedance 7 connect the air-filled chamber 5 with an opening 8 at the rear or at the periphery of the microphone.

It is a disadvantage of all air-filled chambers of such kind together with the described drilled holes to form a Helmholtz-resonator having a resonance frequency within the upper audio frequency range. Though it is possible to damp the resonator in order to do away with its influence on the frequency response of the microphone, nearly in all microphones of that kind the phase-shift of the resonator disturbs, within a definite frequency range, the phase shift of the sound energy coming from the backward opening to the rear of the diaphragm and, by that, disturbs the unidirectional characteristic of the microphone.

Therefore, the polar diagram of all known unidirectional microphones in the frequency region between approximately 6 and approximately 12 kc.p.s. has discontinuities and irregularities, due to the volume of the airfilled chamber.

Above this frequency section, due to the diameter of the microphone, usually the microphone-polar-diagram again becomes better by the influence of the sound diffraction on the diaphragm not yet being small compared with the sound-wavelength and by the influence of soundshadowing-effects.

It is the purpose of the present invention to provide a condenser microphone not having the above-described disadvantages. The back plate of the present microphone is equipped with grooves disposed on its surface and with drilled holes ending in these grooves. By this means the sound energy which reaches the rear of the diaphragm after having passed an acoustical resistance, is conducted directly through a small number of drilled holes of rather large diameter into grooves arranged in the upper side of the back plate and being arranged straight behind the diaphragm, so that the sound energy is distributed very well to the different regions of the rear of the diaphragm.

The local arrangement, the number and the cross-section area of the grooves may be varied over a wide range. The grooves may be straight or curved. They may cross one another; but crossing is not necessary. When crossing one another, the small number of drilled holes may end in the crossing points of the grooves effecting thereby a good distribution of the sound. It is very useful, when the grooves form a line-net spreading all over the upper area of the back plate behind the diaphragm.

The single parts of the sound energy conducted to the rear of the diaphragm will reach the diaphragm by some separated ways without being joined indefinitely with one another. The single components of the sound energy are joined when reaching the zone directly behind the diaphragm. That means, they are joined in a region where in any case it is necessary to combine the difierent forces.

Referring now again to the drawings, and more particularly to FIGS. 3 and 4, an example of a principal arrangement of the microphone according to the present invention, is illustrated, comprising a back plate 3 which has grooves 9 crossing one another. Behind the diaphragm 1' an air slit 2' is provided. The back has six grooves 9 crossing each other. In the crossing points of the grooves 9, drilled holes 10 terminate. The acoustical resistance 11 belonging to the phase shifting network of the microphone, for instance, may be built of textures or wickerwork consisting of textiles, synthetic plastics or wires, or of a porous sintered material. The components of the microphone are mounted in a housing 14 by means of a shell 12 and a clamping ring 13, both elements consisting of non-conductive material.

In FIG. 4 the diaphragm 1 is not shown, to make visible the back plate 3 with the grooves 9 and the drilled holes 10.

Referring now again to the drawings, and in particular to FIG. 5, for example the mentioned acoustical resistance consists of a friction element formed by an air space as slit 17. An acoustical resistance formed in this manner may be built in a simple way. It is easily adjustable and of good stability, when adjusted. A perforated plate 15 is provided and has, for instance, twelve drilled holes 16 and is mounted at a very small distance spaced behind the back plate 3. Its drilled holes 16 are not aligned with the drilled holes 10 of the back plate 3 but are laterally displaced a small distance.

For instance, there is always one drilled hole 16 between two drilled holes 10, respectively, vice versa. The location of the drilled holes 16 behind the back plate 3 in FIG. 4 is clearly illustrated in the drawings.

The width of the slits 17 controls its acoustical resistance and may be adjusted by appropriate profiling of the back plate or of the perforated plates as well as by an intermediate layer, for instance in the form of a circleor ring-shaped foil 18.

The other parts shown in FIG. 5 are identical with those disclosed in FIGS. 3 and 4 and bear the same numerals.

The arrangement of grooves instead of the known airfilled chamber described above, and instead of all similar arrangements, has the great advantage that no resonance effects and no unwanted phase shifts effected by those measures are experienced.

Indeed, a portion of the grooves theoretically is long enough to be effective as a tuned half-wavelength-line tuned in the audio frequency range, provided the dimensions of the microphone are of the commonly used size. Nevertheless, the tuned-line feature is disturbed by several crossing grooves in a way that no resonance effect can be remarked.

Unidirectional microphones constructed in accordance with the present invention in the critical frequency range between six and twelve kilocycles/sec. have the same good front-to-back-ratio as known unidirectional microphones only have in the frequency section below 6 kc.p.s. and above 12 kc.p.s. Moreover, the frequency response of the described microphones in the whole frequency range and for all angles of sound incidence, wherein the microphone receives at least some remarkable sound energy, is of excellent linearity.

Further, a microphone built in accordance with the present invention, may be manufactured by simpler methods than all unidirectional condenser microphones heretofore and its acoustical transducer features are of better reproducibility than for all unidirectional condenser microphones manufactured until now.

While I have disclosed several embodiments of the present invention with certain useful variants thereof, it is to be understood that these embodiments are given by example only and not in a limiting sense, the scope of the present invention being determined by the objects and the claims.

I claim:

1. A unidirectional condenser microphone, comprising an electrically conductive diaphragm,

a back plate metallized at least on its surface,

said electrically conductive diaphragm and said back plate defining an air slit therebetween,

means for connecting and insulating said diaphragm and said back plate and forming an opening at the rear of said microphone,

an acoustical impedance means defined in said connecting means behind said back plate in said opening,

a first electrical terminal secured to said diaphragm,

a second electrical terminal secured to said back plate,

said back plate having holes extending from the surface of said back plate in front of said diaphragm into said acoustical impedance means and from this impedance to said opening at the rear of said microphone,

said back plate having surface grooves distributed over its surface in front of said diaphragm, and

said holes terminating in said grooves.

2. The condenser microphone, as set forth in claim 1, wherein said grooves in said back plate cross each other and are uniformly distributed on said back plate, and said holes terminating in the crossing points of said grooves.

3. The condenser microphone, as set forth in claim 1, which includes a perforated plate disposed behind said back plate and having a plurality of additional holes,

said additional holes being laterally displaced relative to said holes of said back plate.

4. The condenser microphone, as set forth in claim 3, which includes a substantially ring-shaped foil disposed between said back plate and said perforated plate, in order to control the distance between said plates.

5. The condenser microphone, as set forth in claim 1, wherein said acoustical impedance means includes at least one acoustical resistance member disposed adjacent the back side of said back plate.

6. The condenser microphone, as set forth in claim 5, wherein said acoustical resistance member comprises fiber material.

7. The condenser microphone, as set forth in claim 5, wherein said acoustical resistance member comprises porous sintered material.

8. The condenser microphone, as set forth in claim 1, wherein said acoustical impedance means constitutes an air space behind said back plate.

References Cited UNITED STATES PATENTS 1,456,538 5/1923 Crandall 179106 2,910,539 10/ 1959 Hartsfield 179-111 3,118,979 1/1964 Sessler et al. 179-111 3,190,972 6/1965 Schoeps et al. 1791l1 KATHLEEN H. CLAFF Y, Primary Examiner. LAURENCE A. WRIGHT, Assistant Examiner.

US. Cl. X.R. l79180 

